In coding of sound signals such as speech and music, a method of coding an input sound signal in the frequency domain is widely used. As the method of coding a sound signal in the frequency domain, there are, for example, methods of Non-patent Literature 1 and Non-patent Literature 2.
The coding method described in Non-patent Literature 1 is a method that performs coding processing using a spectral envelope based on coefficients which are convertible into linear prediction coefficients. Specifically, the coding method described in Non-patent Literature 1 is a method that obtains a linear prediction coefficient code by coding coefficients which are obtained from an input sound signal and are convertible into linear prediction coefficients and obtains a normalized coefficient code by coding a normalized coefficient sequence which is obtained by normalizing a frequency domain coefficient sequence corresponding to the input sound signal by a spectral envelope coefficient sequence corresponding to coefficients which are convertible into quantized linear prediction coefficients corresponding to the linear prediction coefficient code. The coefficients which are convertible into linear prediction coefficients are, for example, linear prediction coefficients themselves, PARCOR coefficients (partial autocorrelation coefficients), LSP parameters, or the like.
The coding method described in Non-patent Literature 2 is a method that performs coding processing involving differential value variable-length coding by obtaining a differential between the logarithmic value of the average energy of coefficients in each frequency domain obtained by division and the logarithmic value of the average energy of an adjacent frequency domain. Specifically, the coding method described in Non-patent Literature 2 is a method that divides a frequency domain coefficient sequence corresponding to an input sound signal into frequency domains such that the lower the frequencies are, the smaller the number of samples of the frequency domain becomes; the higher the frequencies are, the greater the number of samples of the frequency domain becomes, obtains the average energy of each frequency domain obtained by division, and quantizes the average energy on the logarithmic axes; performs variable-length coding on a differential between the value obtained by quantization and the value obtained by quantizing the average energy of an adjacent frequency domain on the logarithmic axes in a similar manner; and adaptively determines, by using the average energy, which was quantized on the logarithmic axes, of each frequency domain obtained by division, a quantization bit number of each frequency domain coefficient and the quantization step width of each frequency domain coefficient, quantizes each frequency domain coefficient in accordance therewith, and further performs variable-length coding thereon.